Modern wireless devices require some clock reference signals to operate radio communications, broadcast frequency modulation (FM) radio reception, satellite localisation, digital processing or real time tracking. These clocks may have various noise requirements. Cellular communications, for example, will require high-quality, low-noise, high-frequency clocks, typically a few tens of megahertz, whereas real-time or FM radio applications can use low quality and lower frequency clocks, typically a few tens of kHz. Low-noise clocks are power-consumption hungry but are only needed a small portion of the time, whereas low-quality clocks consume much less power but are operated during longer periods of time.
As many modern wireless devices are battery operated, the clock systems enclosed within them should be capable of switching between different operating modes to avoid increasing power consumption, while fulfilling their quality and range of frequencies requirements.
For other devices powered by the mains, the power consumption criteria still remain of importance for environmental reasons, since modern homes use more and more communication devices which are in stand-by mode most of the time. Those communication devices are, for example, Ethernet mains plug-ins, cable-based Ethernet interfaces, television displays and computers and in the near future all kinds of domestic appliances such as washing machines or microwave ovens.
According to a first prior art, two different crystals working at two different frequencies, for example at 32 kHz and at 26 MHz, are used for clock signal generation. One drawback of this first prior art is an increased number of components, and an increased number of pins on an integrated circuit. Another drawback of this first prior art is the requirement to design two oscillators in order to accommodate the two very different frequencies.
According to a second prior art, one single crystal at the highest frequency is used and a divider is added for the stand-by mode. The load capacitance of the crystal and the current biasing of the core oscillator are decreased in order to reduce the power consumption. One drawback of this second prior art is the difficulty to optimize the oscillator to alternatively reach two different and opposite goals, that is to say low noise on the one side and low power on the other side, since the same architecture is used in both cases. In particular, the active part of the oscillator, that is to say notably the transistors making up the amplifier that builds up and sustains the oscillation, is the same, which will lead to oversized transistors for the low power mode.
According to a third prior art, for example described in U.S. Pat. No. 7,005,933, an oscillator can work in two different power modes using respectively two different current sources but with the same architecture. Here again, it is difficult to optimize both modes. Besides, oscillation is made with the help of capacitances and inductances, but there is no crystal.
An object of the present disclosure is to provide an improved oscillator and method of operating an oscillator. In particular, an object of the present disclosure is to alleviate at least partly some of the above-mentioned drawbacks.